On the rotational energy distributions of reactive, non-polar species in the interstellar medium

TL;DR

This paper models the non-equilibrium rotational energy distributions of reactive, non-polar diatomic molecules like C2+ in the interstellar medium, highlighting how chemical reactions influence their rotational populations and spectra.

Contribution

It introduces a kinetic model for predicting non-equilibrium rotational distributions of reactive, non-polar molecules in space, specifically focusing on C2+ in the interstellar medium.

Findings

Predicted strong deviations from local thermodynamic equilibrium for C2+

Calculated a new optical spectrum for C2+

Identified conditions where rotational populations are arrested by chemical reactions

Abstract

A basic model for the formation of non-equilibrium rotational energy distributions is described for reactive, homo-polar diatomic molecules and ions in the interstellar medium. Kinetic models were constructed to calculate the rotational populations of C2+ under the conditions it would experience in the diffuse interstellar medium. As the non-polar ion reacts with molecular hydrogen, but not atomic hydrogen, the thermalization of a hot nascent rotational population will be arrested by chemical reaction when the H2 density begins to be significant. Populations that deviate strongly from the local thermodynamic equilibrium are predicted for C2+ in environments where it may be detectable. Consequences of this are discussed and a new optical spectrum is calculated.